This invention relates fuel system for jet engines or other types of gas turbine engines.
FIG. 1 shows a typical fuel control system 10 for a gas turbine engine such as a jet propulsion engine. The system 10 includes in flow series arrangement a supply of fuel represented by arrow 12 which is fed to a boost pump 14 and then to a high pressure pump 16. From the high pressure pump 16 the fuel flow splits with a first portion flowing to a variable area metering valve 18 and a second portion to a delta-p valve 20 which bypasses fuel back to the upstream side of the high pressure pump 16 to set a fixed differential pressure, xcex94P, across the metering valve 18. From the metering valve 18 the fuel flows through an ecology and pressurizing valve 22 which maintains a pressure level on the downstream side of the metering valve 18. The fuel then flows to a flow meter 24 which measures the fuel flow and provides a signal to the aircraft. From the flow meter 24 the fuel flows through a flow divider 26 and then to the primary and secondary fuel nozzles represented by arrows 28 and 30 which spray the fuel into the combustor of the engine.
A Full Authority Digital Engine Control (FADEC) 32 controls the operation of the engine including the fuel control system 10. In particular, the FADEC 32 adjusts the area of the metering valve 18 so that the fuel flow (Wf) exiting the metering valve is delivered in accordance with the following equation.
Wf=CA(xcex94P)xc2xd
where C is a flow constant and A is the area of the metering valve. Though not shown, a linear variable displacement transducer, (LVDT), is mounted to the metering valve to provide a signal to the FADEC indicative of the position of the metering valve.
Disadvantages to this prior art fuel system are (a) the mechanical complexities required to maintain a constant metering valve differential pressure with varying altitude, (b) precision components like the LVDT are required to control metering area with varying temperature, and (c) the inherent inaccuracy of controlling fuel flow through indirect parameters such as area and pressure.
Accordingly, there is a need for a simplified fuel system for a jet engine that controls fuel flow based on measured fuel flow.
An object of the present invention is to provide a simplified fuel system that controls fuel flow based on measured fuel flow.
Another object of the present invention is to provide a fuel system that is self calibrating.
The present invention meets these objectives providing an a fuel system for delivering fuel from a fuel source to combustor in a gas turbine engine having a pump receiving fuel flow from the fuel source and producing a pressurized fuel flow that flows to a flow meter that measures the fuel flow and generates a signal Wfm thereof. A bypass valve bypasses, to the input side of the pump, a portion of the pressurized fuel flow before it reaches the flow meter. An electronic control unit is included that receives the Wfm signal and in response thereto adjusts the bypass valve until the measured fuel flow equals a predetermined desired fuel flow stored in the electronic control unit.
Thus, a fuel system is provided that control fuel flow based on measured fuel flow. Because it controls on measured fuel flow, a fuel system is simpler and hence more reliable than prior art fuel systems. The fuel system is also self-calibrating as is explained in the specification.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.